Gear Having Improved Surface Finish

ABSTRACT

There is a gear set. The gear set has a) a first gear having a first surface and b) an intermeshing second gear having a second surface. The first and second surfaces each, independently, have an isotropic arithmetic mean roughness, Ra, of about 3 microinches or less and are lubricated. There is also a method for increasing the contact surface-fatigue life of a gear set.

STATEMENT OF GOVERNMENT INTEREST

The invention was made by or under contract with the National Instituteof Standards and Technology (NIST) under contract number 70NANB0H3048.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a gear set. More particularly, thepresent invention relates to a gear set having an improved surfacefinish that enhances load-carrying capability, contact surface-fatiguelife, wear resistance, and performance.

2. Description of the Related Art

A problem encountered with sets of two or more intermeshing gears and/orpinions is wear, and, ultimately, failure. The main failure modes forgears are pitting or micropitting, wear, and scuffing. When a gear andpinion interact, the teeth of one necessarily contact the teeth of theother. Without lubrication, the opposing teeth scratch against, scuff,wear down, pit, and/or crack each other. Lubrication postpones the onsetof these effects. Thus, the better the lubrication, the longer the lifeof the gear.

U.S. Pat. No. 6,732,606 B1 discloses a lubricated gear set with a gearhaving a surface roughness of approximately 5 to 12 microinchesarithmetic mean roughness (Ra). The gear set is disclosed as exhibitingimproved contact fatigue life, wear resistance, and performance.

It would be desirable to have a gear set that exhibits enhancedload-carrying capability, contact surface-fatigue life, wear resistance,and performance.

SUMMARY OF THE INVENTION

According to the present invention, there is a gear set that exhibitsenhanced load-carrying capability, contact surface-fatigue life, wearresistance, and performance.

Further according to the present invention, there is a gear set havinga) a first gear having a first surface and b) an intermeshing secondgear having a second surface. The first and second surfaces each,independently, have an arithmetic mean roughness of about 3 microinchesor less and are lubricated.

Still further according to the present invention, there is a method forincreasing the contact surface-fatigue life of a gear set wherein thegear set has a first gear having a lubricated first surface and anintermeshing second gear having a lubricated second surface. The methodhas the step of providing the first and second surfaces with anarithmetic mean roughness of about 3 microinches or less.

DESCRIPTION OF THE FIGURES

FIG. 1 is a perspective view of one embodiment of a spur gear setaccording to the present invention.

FIG. 2 is a top view of another embodiment of a spiral bevel gear setaccording to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Gears and/or pinions (referred to interchangeably herein) useful in thepresent invention have surfaces having an isotropic (independent ofdirection of measurement) arithmetic mean roughness (Ra) of about 3microinches or less. Preferred surfaces have an Ra of about 0.1 to about3 microinches. Most preferred surfaces have an Ra of about 1 to about 2microinches. Ra can be measured by any standard methods, e.g., stylusprofilometry, non-contact surface reconstruction, and atomic-forcemicroscopy.

The surfaces exhibiting the desired Ra will, at minimum, be thosesurfaces that contact opposing surfaces, e.g., the surfaces of the teethof one gear contacting the surfaces of the teeth of another gear and/orpinion. When the gear set is of an embodiment that does not take theform of intermeshing teeth, e.g., a worm gear, the opposing surfacesthat are in contact will exhibit the desired Ra.

The gear set is lubricated during operation. Any lubricant known in theart may be employed, such as natural or synthetic oils and greases. Gearsurfaces are lubricated to a degree sufficient to prevent prematurewear. Gear surfaces will receive a film lubricant layer that will varydepending on factors such as physical properties of the surface, surfacetexture and roughness, physical properties of the lubricant, andoperating conditions (temperature, pressure, etc.). Gear surfaces in thepresent invention will typically have a lubricant film thickness ofabout 1 microinch to about 50 microinches and more typically from about3 microinches to about 30 microinches. The lubricant film thickness thatforms between the meshing gears is independent of the surface finish ofthe meshing gear components.

An advantage of the present invention is the relatively large lambda (λ)ratios afforded. The lambda ratio is the ratio of lubricant filmthickness to the composite surface roughness. The relatively low surfaceroughness of the gear surfaces in the present invention have the effectof increasing the lambda ratio. As the lambda ratio increases, gearsurface durability and wear resistance increase. Thus, surface lifeincreases. Preferably, gear surfaces will have a lambda value of about0.6 or more. More preferably, gear surfaces will have a lambda value ofabout 1.0 to about 2.0.

Superfinishing may be used to refine the surface roughness of variousgears, thereby increasing the lambda ratio and the gear surfacedurability and load-carrying capability, without requiring any changesin lubricant viscosity, relative gear pitch-line velocity ortemperature.

A semi-empirical model was used to determine the effect of surfaceroughness on friction coefficient. The model was designed to capture theeffects of “as ground” gears with an average surface roughness of 20microinches Ra as well as “superfinished” gears with an average surfaceroughness of 1 microinch Ra. A coarse grain potential molecular dynamicsapproach was used to model the interaction between the lubricant andsubstrate. The optimal gear surface finish, Ra, as estimated using thismolecular dynamics modeling approach, was determined to be from about1.25 to about 3.0 microinches Ra.

FIG. 1 illustrates a spur gear set 10. Spur gear set 10 has a gear 12and a pinion 14. The pinion 14, by convention, is the smaller of the twogears. Spur gear set 10 is used to transmit motion and torque (power)between parallel shafts 16, 18. Gear 12 and pinion 14 have teeth 20 and22, respectively, which are generally straight and radially disposed onthe shafts. FIG. 2 illustrates a spiral bevel gear set 40. Spiral bevelgear set 40 has a gear 42 and a pinion 44 that have intersecting axes(not shown) and are used to transmit torque (power) through an anglechange as well as to achieve a desired gear reduction or augmentation(overdrive) between intersecting shafts (not shown). Gear 42 and pinion44 have teeth 46 and 48, respectively, which are generally beveled andhelical. It is clear that these two examples merely illustratenon-limiting applications for the present invention and that a skilledartisan recognizes that other gear types are amenable to such processingto refine their surface finish for improved performance and durability.

Gears can be fabricated from any known rigid material. Most commonly,gears are fashioned from metals, such as but not limited to, cast iron,steel, aluminum, and brass. Gears may also be fabricated from rigidplastics, such as but not limited to, nylon, vinyl chloride,polypropylene, and high-density polyethylene.

Many methods for finishing gear teeth are known in the art. Methodsinclude but are not limited to gear hobbing and shaving, grinding, andhoning.

In gear hobbing and shaving, a gear is rotated in mesh with a gear-likecutter tool. The gear-like tool has cutting edges that extend up anddown the sides of the teeth parallel to the plane of rotation. This isaccompanied by a relative traverse between the gear and the cutter in aplane parallel to the axis of the gear and the cutter. A surfaceroughness in the range of about 40 to about 80 microinches Ra maytypically be obtained with this method.

In gear grinding, the teeth are ground with a grinding tool. A surfaceroughness in the range of about 15 to about 35 microinches Ra maytypically be obtained with this method.

In gear honing, the gear is rotated in mesh with a gear-shaped hone.Portions of the hone at the gear teeth are fabricated from a plasticmaterial that is relatively hard yet highly resilient. The honingoperation occurs by rotating the hone in mesh with the gear whileproviding a traverse stroke parallel to the axis of the gear. Thisdistributes the finishing action evenly throughout each gear tooth. Asurface roughness of about 15 to about 35 microinches Ra may typicallybe obtained with this method. Fine grit honing may yield a surfaceroughness as low as about 12 to 13 microinches Ra.

A surface roughness of about 3 microinches Ra or less can be obtained byany suitable surface-texture technique, such as but limited to,superfinishing (i.e., chemically accelerated vibratory polishing),etching, chemical polishing, electropolishing, mechanical polishing,honing, and vibratory finishing without chemical enhancement. In someembodiments, chemical and physiochemical methods may be utilized toobtain a surface roughness of about 3 microinches Ra or less. Polishingcompounds used for preparing metal parts for electroplating are useful,e.g., liquid polishing compounds containing fine abrasive particulates.

One suitable superfinishing technique is described in U.S. Pat. No.4,491,500, which discloses a process for refining metal surfaces inwhich a two-step process employing a liquid chemical is followed by aburnishing liquid. A relatively soft coating is formed, which issubsequently treated and physically removed. In the technique, a mass ofelements, comprised of a quantity of objects with hard metal surfaces ofarithmetic average roughness value in excess of about 15, is introducedinto the container of mass finishing equipment. The mass of elements iswetted with a liquid substance capable of rapid reaction, underoxidizing conditions, to chemically convert the metal of the objectsurfaces to a stable film of substantially reduced hardness, and themass is rapidly agitated, while maintaining the metal surfaces in awetted condition with the liquid substance, to produce relative movementand abrasive contact among the elements thereof and to producecontinuous oxygenation of the liquid substance. The reactivity of theliquid substance and the intensity of agitation of the mass arecontrolled to maintain the stable film on the metal surfaces at least atthe level of visual perceptibility. Agitation is continued for a periodsufficient to produce a finish of arithmetic average roughness less thanabout 14, and preferably less than about 10; thereafter, the objectswill generally be treated to dissolve the stable film from the metalsurfaces. In the preferred embodiments of the technique, the mass ofelements introduced into the mass finishing equipment will include aquantity of abrasive finishing media, and the agitation step will becarried out for a period of less than six hours. Generally, the surfaceswill be of a metal selected from the group consisting of iron, copper,zinc, aluminum, titanium, and the alloys thereof, and the stable filmwill comprise an oxide, phosphate, oxalate, sulfate, and/or chromate ofthe substrate metal. Thus, the liquid substance utilized to chemicallyconvert the metal of the object surfaces will usually be a solutioncontaining one or more of the radicals: phosphate, oxalate, sulfate,chromate, and mixtures thereof, and in certain instances it will bepreferred for the substance to additionally include an oxidizing agent;generally, the liquid substance will have an acidic pH value. Solutionscontaining phosphate and oxalate radicals in combination with a peroxidecompound are often found to be particularly effective for refiningferrous metal surfaces, and may be produced from a tripolyphosphatesalt, oxalic acid, and hydrogen peroxide.

Another suitable superfinishing technique is described in U.S. Pat. No.4,818,333, which discloses a process that uses a treatment compositionhaving a high density burnishing media. In the technique, a mass ofelements, including a quantity of objects having relatively rough metalsurfaces, and a solution capable of converting the surfaces to a softerform, are introduced into the container of a mass finishing unit and arerapidly agitated therein to produce relative movement among the elementsand to maintain the surfaces in a wetted condition with the solution,for conversion of any exposed metal, on a continuous basis. A quantityof relatively nonabrasive solid media elements are included, the amountand size of which are such that, under the conditions of agitation,relative sliding movement is promoted among them and with respect to theobjects. The media elements are comprised of a mixture of oxide grains,fused to a coherent mass and substantially free of discrete abrasiveparticles, the coherent mass containing, on an oxygen-free basis, about60 to 80 weight percent aluminum and about 5 to 30 weight silicon. Theywill have a density of at least about 2.75 grams per cubic centimeter(g/cc) and preferably an average diamond pyramid hardness (DPH) value ofat least about 845; taken in quantity, the media elements will have abulk density of at least about 1.70 grams per cubic centimeter.

U.S. Pat. Nos. 4,491,500 and 4,818,333 are incorporated herein byreference in their entirety. Other useful methods are those known in theelectroplating art for obtaining smooth surfaces, such as etching andbright dipping.

Gears useful in the present invention can take various forms, e.g., aspur gear, an internal gear, a helical gear, a herringbone gear, a bevelgear, a worm gear, or a planetary gear, etc.

The following is an example of the present invention.

EXAMPLE

The performance of a spur-gear sets in accordance with the presentinvention was compared to two comparative spur-gear sets that were notsurface treated to a smooth finish.

All three sets of spur gears were composed of Pyrowear alloy 53, analloy that is produced and supplied by Carpenter Technology Corporation.As-ground gears were obtained by grinding. As-ground gears exhibited alambda (λ) of ˜0.3. Intermediate gears were obtained by isotropicsuperfinishing (ISF) the as-ground gears to an intermediate roughnessvalue between those of the as-ground and final superfinished conditions.This was accomplished by subjecting the as-ground gears to a lesser ISFprocessing duration. The finished gears were obtained by ISF processingthe as-ground gears for a greater duration, resulting in the surfaceroughness value presented in the Table. The finished gears exhibited alambda (λ) of ˜1.5.

The three sets of spur gears were tested by assembling a pair of 4-inchtest spur gears, composed of the surface-carburizing alloy, Pyrowearalloy 53, into a test rig. The test rig intermeshed the gear pair andapplied a torque between the meshing gears that produced the surfacecontact stresses shown in the Table. The rotational speed was about 3500revolutions per minute (rpm). The lubricant was 5 cSt polyol ester (POE)conforming to MIL-L-23699 or DoD-PRF-85734 specifications. Oil filmthickness on the gears was ˜6 microinches. The results of these testsare set forth in the Table.

TABLE Surface Pitting- Scoring- Rough- Surface Surface AllowableAllowable ness, Contact Power Life Life Design Improvement ImprovementSurface Ra Stress Increase Np Increase Allowable Factor Factor Condition(μin) (Ksi) Factor (M) Factor (Ksi) Stress Power Stress Power As-Ground*16 235 1.00 16.6 1.0 186 1.00 1.00 1.00 1.00 Intermediate 8 263 1.2524.2 1.5 212 1.14 1.30 1.08 1.18 Roughness* ISF - 2 263 1.25 >61.9 >3.7229 1.23 1.52 1.40 1.95 Processed 280 1.42 37.6 2.3 (Finished) *Not anexample of the present invention Ksi—thousands of pounds per square inchM—millions (of cycles to pitting failure)

The finished gears (e.g. the gears finished to a surface roughness ofless than about 3 microinches Ra via the techniques described herein)exhibited significantly longer surface life, and, concomitantly, greaterlife-increase factor, than the as-ground gears and the intermediateroughness gears. The finished gears of this invention contributed to apower density increase for pitting failures of about 52%. Moreover, thefinished gears produced about a 95% increase in power-density forfailure by scoring (scuffing).

It should be understood that the foregoing description is onlyillustrative of the present invention. Various alternatives andmodifications can be devised by those skilled in the art withoutdeparting from the invention. Accordingly, the present invention isintended to embrace all such alternatives, modifications and variancesthat fall within the scope of the appended claims.

1. A gear set, comprising: a first gear having a first surface and anintermeshing second gear having a second surface, wherein the first andsecond surfaces each, independently, have an arithmetic mean roughnessof about 3 microinches or less, and wherein the first surface and thesecond surface are lubricated.
 2. The gear set of claim 1, wherein thearithmetic mean roughness is about 0.1 to about 3 microinches.
 3. Thegear set of claim 1, wherein the arithmetic mean roughness is about 1 toabout 2 microinches.
 4. The gear set of claim 1, wherein the first andsecond gear have at least one of a spur gear, an internal gear, ahelical gear, a herringbone gear, a bevel gear, a worm gear, and aplanetary gear.
 5. The gear set of claim 1, wherein both the first andsecond gears are spur gears.
 6. The gear set of claim 1, wherein thefirst and second surfaces each have a lambda value of about 1.25 toabout 3.0.
 7. The gear set of claim 1, wherein the first and secondsurfaces each have substantially the same arithmetic mean roughness. 8.A method for increasing the contact surface-fatigue life of a gear setwherein the gear set has a first gear having a lubricated first surfaceand an intermeshing second gear having a lubricated second surface,comprising providing the first and second surfaces with an arithmeticmean roughness of about 3 microinches or less.
 9. The method of claim 8,wherein the arithmetic mean roughness is about 0.1 to about 3microinches.
 10. The method of claim 8, wherein the arithmetic meanroughness is about 1 to about 2 microinches.
 11. The method of claim 8,wherein the first and second gear have at least one of a spur gear, aninternal gear, a helical gear, a herringbone gear, a bevel gear, a wormgear, and a planetary gear.
 12. The method of claim 8, wherein both thefirst and second gears are spur gears.
 13. The method of claim 8,wherein the first and second surfaces each have a lambda value of about1.25 to about 3.0.
 14. The method of claim 8, wherein the first andsecond surfaces each have substantially the same arithmetic meanroughness.